The Concentration Dependence of the Right to Left Conformational Transition in Natural DNA Identified by Raman Spectroscopy

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Abstract
The classical and resonance Raman spectra of DNA from Chicken Erythrocytes have been obtained for different DNA concentrations in solution with low and high ionic strengths. The classical Raman spectra of 30 mg/ml DNA solutions were measured in varying the sodium chloride concentration from 0.1 to 4.5 M NaCl. An increase in the salt content of the solution leads to spectral changes in the 600–700 cm−1 region, indicating a C2′ endo/anti to C3′ endo/syn conformational transition of the purine residues. Other changes around 840 cm−1, due to the antisymmetrical stretching vibration of the PO2 group, are also detected: they were characteristic for the B→-Z transition in model systems such as poly(dG-dC) · poly(dG-dC). The resonance Raman spectra of low (1 mg/ml) and high (30 mg/ml) concentrated DNA solutions were obtained with low (0.1 M) and high (4.5 M) NaCl contents, in using a 284 nm excitation wavelength. No change was observed in the intensities and band positions in the low and high salt solutions of low concentrated DNA Thus it is assumed that the DNA structure remains unchanged whatever the salt concentration for low concentrated DNA In contrast, great modifications of the intensities and positions of some lines were found in the spectra of high DNA concentration solution when the NaCl content is increased up to 4.5 M: these changes resemble to some extent those observed in the study of B→Z transition of several polynucleotide model compounds. It is assumed that the right-handed to left-handed conformational transition may occur in certain sections of natural DNA likely containing alternating purine-pyrimidine sequences, when the DNA concentration is sufficiently important.